1. Field of the Invention
The present invention relates to a slide fastener stringer including a stringer tape and a series of fastener coupling elements sewn to the tape along one longitudinal edge thereof. The invention also relates to a method of manufacturing such slide fastener stringer.
2. Prior Art
It is known to sew a series of fastener coupling elements to a stringer tape by utilizing single thread chain stitches or multi-thread chain stitches, usually formed with a multifilament yarn or yarns.
Single Thread Chain Stitch is formed with a single needle thread and is quite simple in structure, and hence does not require a sophisticated sewing machine. Using this type of stitch enables the coupling elements to be sewn to the tape with maximum ease with minimum cost. Further, this type of stitch has a high degree of stretchability, which is one of the essential factors for a well workable slide fastener. Despite these advantageous characteristics, single thread chain stitch has not been widely used in sewing the coupling elements to the stringer tape. This is because the sewing thread of this stitch type extends over and around the coupling elements only with a small degree of tightness, and because, in the event the thread is broken, the sewing stitches would easily become loose all the way, thereby allowing the coupling elements to be separated apart from the tape.
With such small degree of tightness of the stitches, firm attachment of the coupling elements to the tape is difficult to achieve. FIG. 10 of the accompanying drawings shows, in transverse cross section, a fragment of a pair of interengaged slide fastener stringers 40,41 of the prior art, in which a pair of rows of coupling elements, 42,43 are sewn to a pair of stringer tapes 44,45, respectively, simply by use of single thread chain stitches. Assuming that the fastener stringer 40,41 are sharply bent in the longitudinal direction such that the top surface (undersurface in this Figure) of the tape 44,45 on which the coupling elements 42,43 are disposed becomes concave, the tape edges with the sewing stitches 47,47 are displaced from the normal position toward respective connecting portions 48,48 of the opposed coupling elements 42,42, i.e. from the phantom line position to the solid line position. For this reason, the prior fastener stringers 40,41 would often accidentally split open when they are bent.
Multi-thread Chain Stitch, which is also known as "Double Locked Stitch", is formed with two or more sewing threads, i.e. needle and looper threads, and has a complicated structure, and hence can prevent the sewing threads from becoming loose all the way even when some of the sewing threads are broken. Using this type of stitch, the coupling elements can be held more stably and firmly in position on the tape. On the other side, multi-thread chain stitch requires an increased amount of sewing thread, as compared with single thread chain stitch, and involves complicated sewing operation which is time-consuming and must be accomplished by a specially designed sewing machine. Moreover, using multi-thread chain stitch necessarily spoils the flexibility of the stringer tape, which is one of the essential factors for a slide fastener product free from accidentally splitting open when subject to bending forces.
In the prior slide fastener stringer, loops of the sewing threads are disposed over those legs of the coupling elements which are remote from the tape, no matter which type of stitch is used. With such arrangement of the thread loops, a slider cannot be moved smoothly along the opposed rows of coupling elements, and hence the sewing threads are liable to be worn out. Ironing is another cause of the breakage of the sewing threads.